Stratix GX Architecture
PLL typically provides 0.5% down spread modulation using a triangular
profile. The modulation frequency is programmable. Enabling spread
spectrum for a PLL affects all of its outputs.
Lock Detect
The lock output indicates that there is a stable clock output signal in
phase with the reference clock. Without any additional circuitry, the lock
signal may toggle as the PLL begins tracking the reference clock. You may
need to gate the lock signal for use as a system control. The lock signal
from the locked port can drive the logic array or an output pin.
Whenever the PLL loses lock for any reason (be it excessive inclk jitter,
clock switchover, PLL reconfiguration, power supply noise etc.), the PLL
must be reset with the aresetsignal for correct phase shift operation. If
the phase relationship between the input clock versus output clock, and
between different output clocks from the PLL is not important in the
design, then the PLL need not be reset.
f
See the Stratix GX FPGA Errata Sheet for more information on
implementing the gated lock signal in the design.
Programmable Duty Cycle
The programmable duty cycle allows enhanced PLLs to generate clock
outputs with a variable duty cycle. This feature is supported on each
enhanced PLL post-scale counter (g0..g3, l0..l3, e0..e3). The duty cycle
setting is achieved by a low and high time count setting for the post-scale
dividers. The Quartus II software uses the frequency input and the
required multiply or divide rate to determine the duty cycle choices.
Advanced Clear & Enable Control
There are several control signals for clearing and enabling PLLs and their
outputs. You can use these signals to control PLL resynchronization and
gate PLL output clocks for low-power applications.
The pllenablepin is a dedicated pin that enables/disables PLLs. When
the pllenablepin is low, the clock output ports are driven by GNDand
all the PLLs go out of lock. When the pllenablepin goes high again, the
PLLs relock and resynchronize to the input clocks. You can choose which
PLLs are controlled by the pllenablesignal by connecting the
pllenableinput port of the altpllmegafunction to the common
pllenableinput pin.
Altera Corporation
February 2005
4–91
Stratix GX Device Handbook, Volume 1
ALTERA [ ALTERA CORPORATION ]
